HONG KONG — The police in Hong Kong began tightening on Sunday a ring of security around thousands of pro-democracy protesters who have besieged the city government for three days. But by clearing the protesters and appeasing the Chinese Communist Party, the Hong Kong authorities could risk a bigger backlash from even more city residents, said experts.

The Hong Kong government has been grappling with how to defuse the sit-in protest that started on Friday night and stretched over the weekend, swelling at times to a crowd of tens of thousands. Although the police had been practicing for months to quell planned protests over election rule changes, they failed on Friday to prevent hundreds of students from charging into a forecourt at the city government headquarters, drawing many more supporters who occupied an avenue and open areas next to the fenced-in forecourt. The students inside the forecourt were dragged off by the police on Saturday, but the supporters outside have stayed.

On Sunday afternoon, the police began to seal off the sit-in area, stopping supporters from entering. The city leader, Leung Chun-ying, told a news conference that the protesters were using illegal methods to threaten the government, and he declared his “absolute trust in the professional judgment of the police.”

But Benny Tai, an associate professor of law at the University of Hong Kong and co-founder of the main group behind the pro-democracy campaign, said he and other protesters were prepared to stay and peacefully resist any effort to clear the area, which many of them call “Civic Square.”

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An injured protester was tended to after clashing with the riot police on Saturday in Hong Kong.CreditLam Yik Fei/Getty Images

“At this stage, we will defend Civic Square with citizens and students until the last minute,” Mr. Tai said in an interview, his voice raspy after two days of constant speeches and interviews. He said that the Hong Kong government’s response would probably be guided by advice and signals from Beijing, which exercises sovereignty over the city.

“It’s hard for me to guess what the Chinese government thinks,” he said. “A responsible government that loves its people would be moved and touched. But I’m not sure they love their own people.”

Several democratic politicians in Hong Kong said the unexpected strength of the young protesters had persuaded an older generation to cede more influence to student activists, who seem less open to compromise with authoritarian Beijing.

“What happened since yesterday was beyond our expectation,” Albert Ho, 62, a member of the Democratic Party in Hong Kong’s Legislature, said in an interview late Saturday.

“Now the younger people have taken control and used their advantage of surprise,” Mr. Ho said at an exuberant rally attended by thousands of people, mostly in their 20s or younger, in front of the city government offices. “This is something that will deeply concern the government.”

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Students staged a sit-in outside the Central Government Offices in Hong Kong on Saturday.CreditJerome Favre/European Pressphoto Agency

The power of the young activists was underlined when Occupy Central with Love and Peace, Mr. Tai’s group, announced an abrupt change in its plans for civil disobedience protests in the city’s main financial district, known as Central. Those demonstrations, which the organizers said were likely to start on Wednesday, would oppose election proposals issued last month by the Chinese government that the campaign says do not offer authentic democratic options for choosing the city’s chief executive.

In the early hours of Sunday, Mr. Tai announced that the students’ “occupation” in front of the city government offices would for now be the base for his group’s protests as well.

“Students have activated Hong Kong’s largest-scale civil disobedience campaign ever,” Mr. Tai said from a small stage, in a speech punctuated by roaring cheers and shouted slogans. He said the group was still examining how it might stage protests in Central, as it first planned. “We’ll begin,” he said, “by occupying the central government offices.”

Throughout the weekend, protesters had turned the government complex beside Victoria Harbour into a raucous but orderly camp. The protests started after a weeklong boycott of classes by university students to protest China’s election plan.

“We think that this place is ours, not the government’s,” said Will Mak Wing-kai, a student from the Chinese University of Hong Kong.

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Protesters use barricades to block the road during a rally.CreditLam Yik Fei/Getty Images

“I want the government to be representative, elected by us from our hearts, not by the Chinese government,” Mr. Mak said, rapidly taking phone calls about organizing the swelling crowd. Under current electoral laws, the chief executive is selected by a committee dominated by Beijing loyalists.

On Saturday, the crowd grew into many thousands, and veered from angry jeering to an almost celebratory mood when the number of police officers thinned. Official agencies offered no estimates of crowd totals.

Many in the crowd — fearing that the police would use pepper spray, as they had on Friday night and Saturday morning — unfurled umbrellas, donned plastic raincoats and gauze masks, and put plastic wrap over their eyes. Some wore yellow ribbons symbolizing hope for change. Seventy-four people have been arrested since the confrontation started on Friday, the police said Saturday.

Some older protesters said the sight of the police squirting pepper spray at the students, which was shown on television news reports, galvanized support for the demonstration. In a society that reveres education, the students have drawn an outpouring of donations from residents, who sent bottled water, tissues and snacks, which accumulated into mountains of supplies.

Some sensed echoes from Beijing 25 years ago, when the students who occupied Tiananmen Square received public support and donations before the protests were brutally suppressed.

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Thousands of protesters attend a rally outside the government headquarters in Hong Kong.CreditTyrone Siu/Reuters

“They are ready to pick up the democracy baton from the student movement in China in 1989,” said Sunny Lau, 57, who said he had been pepper-sprayed by the police at this weekend’s protests.

Since Hong Kong returned to Chinese sovereignty in 1997, the former British colony has kept its independent courts and legal protections for free speech and assembly, as well as a robust civil society. But many democratic groups and politicians say those freedoms have eroded under mainland China’s growing political and economic influence.

The current chief executive, Mr. Leung, has backed Beijing’s plan for electoral changes, which for the first time would let the public vote for the city’s leader, starting in 2017. But critics say the plan includes procedural hurdles that would screen out candidates who do not have Beijing’s implicit blessing, making the popular vote meaningless.

Anger with the Chinese government runs especially deep among Hong Kong residents in their 30s and younger, according to polls. Younger residents feel squeezed by rising housing prices and living expenses and lack of upward mobility, and they often accuse the government of pandering to tycoons.

“I think unfairness is spreading in Hong Kong, and because of the political system,” said Edith Fung, 21, a student of land surveying. “I don’t want Hong Kong to change to be like China, with corruption, unfairness, no press freedom, no religious freedom.”

Ms. Fung said she had been indifferent to politics until this year, and Eva Mo, a nursing student who helped provide first aid to protesters, said she was participating in her first demonstration against the government. “They treated the students like children,” she said. “We only asked for dialogue. But there was none.”

A version of this article appears in print on , on Page A6 of the New York edition with the headline: Pro-Democracy Group Shifts to Collaborate With Student Protesters in Hong Kong. Order Reprints | Today’s Paper | Subscribe